Navigator assert on non restorable routes returned by onGenerateInitialRoutes (#73082)

packages/flutter/lib/src/widgets/navigator.dart

diff --git a/packages/flutter/lib/src/widgets/navigator.dart b/packages/flutter/lib/src/widgets/navigator.dart
index 640832a..4beefed 100644
--- a/packages/flutter/lib/src/widgets/navigator.dart
+++ b/packages/flutter/lib/src/widgets/navigator.dart
@@ -3388,6 +3388,13 @@
       }
     }
 
+    assert(
+      _history.isNotEmpty,
+      'All routes returned by onGenerateInitialRoutes are not restorable. '
+      'Please make sure that all routes returned by onGenerateInitialRoutes '
+      'have their RouteSettings defined with names that are defined in the '
+      'app\'s routes table.',
+    );
     assert(!_debugLocked);
     assert(() { _debugLocked = true; return true; }());
     _flushHistoryUpdates();

packages/flutter/test/widgets/navigator_restoration_test.dart

diff --git a/packages/flutter/test/widgets/navigator_restoration_test.dart b/packages/flutter/test/widgets/navigator_restoration_test.dart
index 3142e8e..f17e6ee 100644
--- a/packages/flutter/test/widgets/navigator_restoration_test.dart
+++ b/packages/flutter/test/widgets/navigator_restoration_test.dart
@@ -981,6 +981,44 @@
     await tester.pumpAndSettle();
     expect(findRoute('p1', count: 0), findsOneWidget);
   });
+
+  testWidgets('Helpful assert thrown all routes in onGenerateInitialRoutes are not restorable', (WidgetTester tester) async {
+    await tester.pumpWidget(
+      MaterialApp(
+        restorationScopeId: 'material_app',
+        initialRoute: '/',
+        routes: <String, WidgetBuilder>{
+          '/': (BuildContext context) => Container(),
+        },
+        onGenerateInitialRoutes: (String initialRoute) {
+          return <MaterialPageRoute<void>>[
+            MaterialPageRoute<void>(
+              builder: (BuildContext context) => Container(),
+            ),
+          ];
+        },
+      ),
+    );
+    await tester.restartAndRestore();
+    final dynamic exception = tester.takeException();
+    expect(exception, isAssertionError);
+    expect(
+      (exception as AssertionError).message,
+      contains('All routes returned by onGenerateInitialRoutes are not restorable.'),
+    );
+
+    // The previous assert leaves the widget tree in a broken state, so the
+    // following code catches any remaining exceptions from attempting to build
+    // new widget tree.
+    final FlutterExceptionHandler? oldHandler = FlutterError.onError;
+    dynamic remainingException;
+    FlutterError.onError = (FlutterErrorDetails details) {
+      remainingException ??= details.exception;
+    };
+    await tester.pumpWidget(Container(key: UniqueKey()));
+    FlutterError.onError = oldHandler;
+    expect(remainingException, isAssertionError);
+  });
 }
 
 Route<void> _routeBuilder(BuildContext context, Object? arguments) {